In U.S. Pat. No. 4,098,943, there is disclosed a process for manufacturing multi-ply sheets involving two webs of plastics material supplied by two extruders. A polishing stack is downstream of each extruder for polishing the webs which then are fed to a further set of rolls. A third extruder supplies a thermoplastic sheet which is fed, together with a long glass mat supplied from rolls, to the same set of laminating rolls. When entering thereinto, the thermoplastic sheet has a temperature above the fusion point of the plastics material so that the glass mat may become encased in the thermoplastic material. The two webs mentioned initially are to be joined in the process, though they are chilled to a larger extent after having passed the polishing stack which is arranged at a relatively large distance from the set of rolls downstream of which further smoothing rolls are arranged. From the webs thus assembled, the multi-ply sheets are cut to desired dimensions by means of suitable tools.
It will be seen that there are circumstances involved in performing this method which requre comparatively larger expenditures, the more so since an additional extruder is necessary for embedding the glass mat or any type of structured web in the multi-ply sheets being produced. Moreover, a smoothing or polishing stack is arranged downstream of each of the two other extruders. Further problems arise with respect to control and, in particular, to synchronizing the operation of the various units such as the set of rolls, two polishing stacks and three extruders.